When Is Pumice Stone Formed?

Pumice stone is formed when molten volcanic glass, ejected from beneath the earth’s crust, cools so rapidly that there is no time for it to crystallize. After the pumice has solidified, the gases inside are suddenly released and the stone swells up into its characteristic light and airy form.

If the substance had cooled under greater pressure it would have turned into solid glass. The stones have long been used for cleaning and polishing. Since the Second World War it has been employed widely in railroad building, masonry and insulation.

Pumice, called pumicite in its powdered or dust form, consists of highly vesicular rough textured volcanic glass, which may or may not contain crystals. It is typically light colored. Pumice is created when super-heated, highly pressurized rock is violently ejected from a volcano. The unusual foamy configuration of pumice happens because of simultaneous rapid cooling and rapid depressurization.

The depressurization creates bubbles by lowering the solubility of gases (including water and CO2) that are dissolved in the lava, causing the gases to rapidly exsolve (like the bubbles of CO2 that appear when a carbonated drink is opened). The simultaneous cooling and depressurization freezes the bubbles in a matrix. Eruptions under water are rapidly cooled and the large volume of pumice created can be a shipping hazard for cargo ships.

Good pumice is found in Iceland, the Canaries, New Zealand, Greece, the Pacific coast of the United States and many other areas with a volcanic background. In United States, Pumice is commercially mined in southwest corner of Idaho by Hess Pumice. Pumice exists in the Kamchatka Peninsula area of Russia where it is known as the volcanic belt.

Pumice is widely used to make lightweight concrete or insulative low-density cinder blocks. When used as an additive for cement, a fine-grained version of pumice called pozzolan is mixed with lime to form a light-weight, smooth, plaster-like concrete. This form of concrete was used as far back as Roman times. Roman engineers used it to build the huge dome of the Pantheon and as construction material for many aqueducts.

It is also used as an abrasive, especially in polishes, pencil erasers, and the production of stone-washed jeans. In personal care, pumice is used as a skin exfoliants. “Pumice stones” are often used in beauty salons during the pedicure process to remove dry and excess skin from the bottom of the foot as well as calluses. It was also used in ancient Greek and Roman times to remove excess hair. Finely ground pumice is added to some toothpastes and heavy-duty hand cleaners (such as Lava soap) as a mild abrasive. Some brands of chinchilla dust bath are made of powdered pumice.

Content for this question contributed by Kathy Misuir, resident of Roselle Park, Union County, New Jersey, USA